Estimation of minor-to-moderate conductive hearing loss with distortion-product otoacoustic emissions in humans.

Distortion-product otoacoustic emission (DPOAE) level maps provide a three-dimensional, frequency-specific representation of cochlear nonlinearities and offer an objective tool to estimate conductive hearing loss (CHL). While previous experimental studies have simulated or transiently induced conductive loss in normal-hearing (NH) ears, this study presents the estimation of the air-bone gap (ABG) based on pulsed DPOAE level maps in patients with pathologic minor-to-moderate CHL or mixed hearing loss. Using an adaptive measurement algorithm, DPOAE level maps were recorded in nine CHL patients and compared with maps from 15 NH controls and five patients with sensorineural hearing loss (SNHL). We evaluated two DPOAE-based metrics: (1) the shift of the optimal L1-L2 stimulus path, and (2) the slope of the semilogarithmic DPOAE growth function. In CHL ears, DPOAE growth slopes decreased and level maps shifted to higher L1, consistent with reverse and forward transmission losses of the middle ear. These features were clearly different in SNHL, enabling differentiation between conductive and cochlear components. Both metrics significantly correlated with the measured ABG, and their average yielded the most accurate ABG estimate. Furthermore, estimated DPOAE thresholds (EDPT) closely tracked behavioral thresholds across groups. DPOAE level maps provide a foundation for separating conductive and sensorineural components objectively. The results support the hypothesis that the slope of the semilogarithmically scaled DPOAE input-output function reflects the CHL and suggests potential utility as an adjunctive tool for diagnosing suspected otosclerosis and for monitoring this pathology.